UWorld

Question 1

Organisms that have low infectious dose (ID50)?

Answer 1

1. Shigella flexneri
2. Campylobacter jejuni (500 organisms)
3. Entamoeba histolytica (1-10)
4. Giardia lamblia (1-10)
5. Shiga toxin-producing E coli
6. Yersinia pestis (<10 organisms)

Question 2

Efflux pumps?

Answer 2

Efflux pumps are generally globular proteins with multiple domains. They reside in the bacterial cell membrane and excrete toxic substances from the interior of the cell. Bacteria can use efflux pumps to generate antibiotic resistance to drugs that operate in the interior of the cell, such as fluoroquinolones and aminoglycosides.

Question 3

Differential media?

Answer 3

Differential media help identify cultured organisms based on their metabolic and biochemical properties. Examples of differential media include the McConkey and eosin methylene blue (EMB) agars used to culture enteric organisms. Organisms that derment lactose will appear pink on MacConkey agar and black on EMB agar.

Question 4

Enrichment media?

Answer 4

Enrichment media contain special growth factors required for some organisms. Ecamples include the X and V factors required by Haemophilus or the anaerobic conditions needed by Clostridium species.

Question 5

Reducing media?

Answer 5

Reducing media (eg, thioglycolate broth) remove oxygen and are used to culture anaerobic organisms.

Question 6

Neutropenic fever?

Answer 6

Defined as a temperature >38.3 and an absolute neutrophil count <500/mm3. Neutropenic fever without an identifiable source (eg, normal chest x-ray, normal urine culture, normal examination) is believed to be caused by translocation of endogenous bacteria into the bloodstream from mucositis, which results from chemoterphy-induced disruption of the gastrointestinal epithelium.

Question 7

Causative pathogenes of neurtopenic fever?

Answer 7

Gram-negative bacterial infections are frequently identified as the causative pathogen of neutropenic fever and are associated with a rapid clinical deterioration due to the production of endotoxins, which stimulate the widespread release of proinflammatory mediators (eg, tumor necrosis factor) that results in hemodynamic decompensation.
Gram-positive infections (eg, from indwelling catheters) also occur but tend to be less virulent with a slower onset and less severe course.

Question 8

Major virulence factors of Bardetella pertussis?

Answer 8

1. Tracheal cytotoxin, which directly damages and destroys ciliated epithelial cells, leading to a loss of airway clearance, subsequent microaspirations, and the development of the characteristic paroxysmal cough.
2. Pertussis toxin, which is an AB toxin that enters cells and activates adenylate cyclase, leading to alterations in cellular signaling that inhibit phagocyte activity and can cause lymphocytosis.

Question 9

Which pathogen`s pathogenesis involved in ‘‘Intracellular invasion of reticuloendothelial cells’’?

Answer 9

Measles infection begins in the respiratory epithelium and spreads via the bloodstream to reticuloendothelial cells.

Question 10

The presence of leukocytes with monocytic predominance is typical for …?

Answer 10

typhoid fever caused by Salmonella Typhi.

Question 11

Leukocytes with neutrophilic predominance?

Answer 11

Invading bacteria that cause dysentery or inflammatory diarrhea (eg, Shigella species, Salmonella species, Campylobacter jejuni, enteroinvasive E coli [EIEC]) are usually marked by leukocytes with neutrophilic predominance and erythrocytes.

Question 12

Erythrocytes without leukocytes?

Answer 12

A hemorrhagic organism (eg, enterohemorrhagic E coli O157:H7, which causes toxin-mediated enterocyte death and intestinal hemorrhage without mucosal invasion) would be characterized by erythrocytes without leukocytes.

Question 13

Triad of septic arthritis in disseminated gonococcal infection (DGI)?

Answer 13

Polyarthiritis, dermatitis (a vesiculopustular skin rash), and tenosynovitis

Question 14

Tularemia?

Answer 14

Tularemia is caused by the gram-negative bacterium Francisella tularensis, which frequently infects rabbits. Manifestation typically include flulike symptoms followed by lymphadenopathy and a papulo-ulcerative eschar at the site of inoculation.

Question 15

What is interference?

Answer 15

Interference occurs when one virus inhibits replication and/or release of a second virus that is infecting the same cell.

Question 16

Differences of viral meningitis from bacerial and fungal meningitis?

Answer 16

Aseptic meningitis is generally less severe, and focal neurologic signs, seizures, and alterations in mental status are absent. The presence of any of these should prompt consideration of other conditions, including bacterial meningitis, encephalitis, and intracranial hemorrhage.

Question 17

Decreased glycogen concentration in the vaginal epithelium is typical in …?

Answer 17

postmenopausal and lactating women. This occurs due to low estrogen levels and can result in atrophic vaginitis (eg, vaginal dryness, pruritis).

Question 18

Nontreponemal serologic test?

Answer 18

Nontreponemal serologic tests (rapid plama reagin, VDRL) mix a patients serum with a cardiolipin-cholesterol-lecithin antigen. Aggregation or flocculation (clumping) indicates the presence of anticardiolipin antibodies in the patients serum. These antibodies form in response to the release of lipids from cells damaged by T pallidum.

Question 19

Clinical manifestations of blastomycosis in immunocompromised patients?

Answer 19

In immunocompromised patients, blastomycosis can cause disseminated disease. Patients experience systemic symptoms (fever, weight loss, night sweats), lung involvement (cough, dyspnea), skin lesions (papules, pustules, ulcers, verrucous lesions), and bone pain (caused by lytic lesions).

Question 20

Differences between influenza and the common cold?

Answer 20

Although both influenza and the common cold can be associated with upper respiratory symptoms (eg, rhinorrhea, cough, sore throat), influenza can often be differentiated based on the abrupt onset of symptoms and presnece of nonspecific, systemic manifestations (eg, fever, malaise, myalgias, headache).

Question 21

Mechanism of action of alcohol-based disinfectants?

Answer 21

Alcohol-based disinfectants, which are composed of ethyl or isopropyl alcohol (at concentrations of 60%-90%) in water, are often used for hand hygiene. These solutions kill vegetative bacteria (but not spores), fungus, and enveloped viruses (eg, influenza, HIV, herpes) by dissolving their lipid bilayer membranes and subsequently denaturing their proteins. Nonenveloped viruses as adenovirus, rhinovirus, enterovirus, and poliovirus are less susceptible to some alcohol-based disinfectants (particularly isopropyl-based solutions) because they do not have a lipid bilayer envelope.

Question 22

Which virus expresses regulatory latency-associated transcript?

Answer 22

Herpes simplex virus type 1 establishes a life-long latent infection within sensory neurons where it expresses latency-associated transcript, a non-protein coding RNA molecule that suppresses viral lytic genes and inhibits host cell apoptosis.

Question 23

Diagnosis of hookworm (Necator americanus and Ancylostoma duodenale)?

Answer 23

Diagnosis is generally made when smooth, thin-walled hookworm eggs are identified by stool microscopy. Peripheral eosinophilia is also an important diagnostic clue.

Question 24

Diagnosis of lymphatic filariasis (Wuchereria bancrofti)?

Answer 24

Diagnosis is made by serology or by identifying worms in the lymphatic system.

Question 25

Antigenically hypervariable pili are characteristic of …?

Answer 25

Neisseria meningitidis and neisseria gonorrhoeae.

Question 26

Nonpurulent cellulitis?

Answer 26

Nonpurulent cellulitis is characterized by skin warmth, edema, and erythema with no fluctuant nodules. It is most often caused by beta-hemolytic streptococci; group A streptococcus (S pyogenes) acoounts for the majority of cases.

Question 27

Purulent cellulitis?

Answer 27

Purulent cellulitis is characterized by a painful, fluctuant nodule in the dermis or subcutaneous tissue with or without surrounding erhtyema. The most common cause is S aureus; strains that express the virulence factor Panton-Valentine leukocidin are particularly likely to cause skin abscess.

Question 28

Urine can harbor pathogenes such as …?

Answer 28

Cytomegalovirus, adenovirus, E coli, S saprophyticus, and Klebsiella pneumoniae

Question 29

Clinical manifestations in early HIV?

Answer 29

Most paients infected with HIV develop mononucleosis-like symptoms (eg, fever, diffuse lympadenopathy, malaise, myalgia, sore throat, and headache) 2-4 weeks after inoculation (acute retroviral syndrome). The presence of oropharyngeal ulcers and a diffuse maculopapular rash are importatnt diagnostic clues.

Question 30

Normal vaginal discahrge?

Answer 30

A wet mount with epithelial cells and rare leukocytes is consistent with normal vaginal discharge (ie, physiologic leukorrhea).

Question 31

Clinical manfestations of acute Q fever?

Answer 31

Acute Q fever manifests as a nonspecific febrile illness with fever lasting >10 days, fatigue, and myalgias. Severe debilitating headaches, which are often retroorbital and associated with photophobia, are a frequent symptom. Pneumonia is one of the primary signs of acute Q fever, and the most common x-ray abnormality is lobar consolidation. Leukocyte count is often normal. Other classic laboratory findings are increased liver enzymes and thrombocytopenia.

Question 32

Virus-like particles are used in which vaccines?

Answer 32

In the hepatitis B and human papillomavirus vaccines.

Question 33

Hidradenitis suppurative?

Answer 33

A chronic follicular occlusive disease that manifests in the intertriginous skin areas (eg, groin, axilla, inframammary). Manifestations usually include recurrent inflammatory nodules and abscesses with pain and malodorous discharge.

Question 34

Vertebral osteomyelitis shold be suspected in patients with …?

Answer 34

new or worsening back pain, fever, and recent endocarditis or bacteremia (especially S aureus). It should also be suspected if there are new neurologic findings and fever, with or without back pain.

Question 35

Serotypes of chlamydia trachomatis?

Answer 35

1. Serotypes A through C cause ocular infection (trachoma) in children;
2. Serotypes D through K cause urogenital (sexually trasmitted) infections and inclusion conjunctivitis;
3. Serotypes L1 through L3 cause lymphogranuloma venereum.

Question 36

Mechanism of action of mefloquine?

Answer 36

Mefloquine is a schizonticide that actively destroys replicating parasites within red blood cells. However, it is inactivated in the liver and has no efficacy against hepatic (exoerythrocytic tissue) schizonts. Because P falciparum schizonts mature in the liver over 8-30 days, travelers must take mefloquine chemoprophylaxis for 4 weeks upon returning from malaria-endemic areas to ensure that schizonts released from the liver are destroyed when they infect red blood cells. Individuals who do not take mefloquine upon return can develop intraerythrocytic infection, leading to symptomatic malaria.

Question 37

Clinical manifestations in fetal infection with parvovirus?

Answer 37

Fetal infection with parvovirus can lead to interruption of erythropoiesis, causing profound anemia and congestive heart failure. Fetal congestive heart failure can cause pleural effusions, pericardial effusions, and ascites. The findings on fetal autpsy (pleural effusion with secondary pulmonary hypoplasia, and ascites) represent fetal hydrops.

Question 38

Mechanism of action of toxins A and B of Clostridioides difficile?

Answer 38

The toxins bind specific receptors on intestinal mucosal cells and are internalized, allowing them to inactivate the Rho-regulatory proteins involved in the maintenance of actin cytoskeletal structure. The result is loss of cytoskeleton integrity, leading to cell rounding/retraction, disruption of intercellular tight junctions, and increased paracellular intestinal fluid secretion (watery diarrhea). Both toxins also have inflammatory effects (eg, neutrophil recruitment) and can induce apoptosis, resulting pseudomembrane formation.

Question 39

Functions of latency genes of Epstein-Barr virus?

Answer 39

Initial infection is followed by life-long latent infection in memory B cells. While latent, the EBV genomes exists as an episome in the nucleus of infected cells and expresses a restricted subset of EBV genes (latency genes) which do the following:
1) Promote periodic viral reactivation from memory B-cells, leading to recurrent (asymptomatic) mucosal infections.
2) Promote survival of infected memory B cells by encoding for proteins (eg, EBNA-1, LMP1) that prevent apoptosis. In a minority of patients, these proteins are oncogenic and lead to malignant transformation of infected B cells (eg, Burkitt lymphoma, Hodgkin lymphoma) or premalignant epithelial cells (eg, nasopharyngeal carcinoma, salivary cancer).

Question 40

Abortive viral infection?

Answer 40

An abortive viral infection refers to an infection in which the virus enters the cell but does not successfully produce new infective virus. Therefore, an abortive infection is not capable of causing disease.

Question 41

Immune defense against Giardia lamblia?

Answer 41

Involves CD4+ T helper cell induction od secretory Ig A production. Secretng IgA helps prevent and clear infection by binding to trophozoites and impairing their adherence to the upper small-bowel mucosa. Children with IgA deficiency, X-linked agammaglobulinemia, and common variable immune deficiency have a predisposition to developing chronic giardiasis.

Question 42

Pathologic manifestetions in giardiasis?

Answer 42

Giardia lamblia causes injury to the duodenal and jejunal mucosa by adhering to the intestinal brush border and releasing molecules that induce a mucosal inflammatory response. Small-bowel biopsy is sometimes performed when stool studies are nondiagnostic and can show varying degrees of villous atrophy and crypt hyperplasia depending on disease severity.

Question 43

Mechanisms of rotavirus-induced diarrhea?

Answer 43

1. Villous blunting (loss of absoptive capacity);
2. Proliferation of secretory crypt cells (secretory diarrhea);
3. Reduced brush border enzymes (accumulation of unmetabolized disaccharides in the small intestine lumen).

Question 44

Musculoskeletal features of early congenital syphilis?

Answer 44

1. Bilateral and symmetric metaphyseal erosions and periosteal inflammation of long bones;
2. Pathologic fractures;
3. Swelling, pain, and limited movement of affected extremities.

Question 45

Clinical manifestations of cutaneous leishmaniasis?

Answer 45

Cutaneous leishmaniasis is characterized by a chronic, enlarging, pinkish papule at the site of the bite that eventually develops into a nodule or plaque and ulcerates.

Question 46

Histopathologic findings in sporotrichosis?

Answer 46

Biopsy of the lesions reveals mixed inflammation, including granulomas and neutrophilic microabscesses in the dermis and subcutaneous tissue.

Question 47

Coccidioides immitis can present in five ways … ?

Answer 47

1. Acute pneumonia (most common);
2. Chronic progressive pneumonia;
3. Pulmonary nodules and cavites;
4. Extrapulmonary nonmeningeal disease;
5. Meningitis.

Question 48

Affinity for erythrocytes of merozoites from different species of malaria?

Answer 48

1. Plasmodium vivax and Plasmodium ovale primarily infect very young, large erythrocytes recently released from the bone marrow (ie, reticulocytes);
2. Plasmodium malariae primarily infects old erythrocytes that have been in the bloodstream for month.
3. Plasmodium falciparum has the unique ability to infect erythrocytes of all ages.

Question 49

Unique ability of Plasmodium falciparum?

Answer 49

P falciparum has a unique ability to significantly increase cytoadherence molecules on the surface of infected erythrocytes, leading to increased risk for microvascular adhesion with subsequent tissue ischemia. The combination of a high degree of parasitemia and a high risk for microvascular adhesion make P falciparum the most severe and deadly species of malaria.

Question 50

Common features of all Plasmodium species?

Answer 50

1. All Plasmodium species that cause malaria rely primarily on anaerobic metabolism bacause they are unable to perform oxidative phosphorylation in their mitochondria.
2. All Plasmodium species that cause malaria digest hemoglobin in vacuoles to supply the parasite with amino acid. This generates hemozoin, a toxic metabolite identified by macrophages in the reticuloendothelial system, leading to erythrocyte pruning and, often, splenomegaly.

Question 51

First-line treatment in toxoplasmosis?

Answer 51

First-line treatment includes a combination of pyrimethamine and sulfadiazine (or pyrimethamine and clindamycin in case of sulfa drug hypersensiticity) along with leucovorin (folinic acid).

Question 52

Pulmonary actinomycosis?

Answer 52

Pulmonary actinomycosis is usually caused by aspiration, which often leads to lower lobe consolidation with air bronchograms (air-filled bronchi with surrounding alveolar opacification). Patients with alcohol use disorder are at increased risk.

Question 53

Pathologic manifestation of ecthyma gangrenosum?

Answer 53

Perivascular bacterial invasion of arteries and veins in the dermis and subcutaneous tissue, with subsequent release of exotoxins destructive to human tissue, leads to characteristic skin patches exhibiting necrosis and ulceration as a result of insufficient blood flow.

Question 54

Pseudomonas aeruginosa virulence factors that may contribute to ecthyma gangrenosum?

Answer 54

1. Exotoxin A (protein synthesis inhibition);
2. Elastase (degrades elastin - important for blood vessel destruction);
3. Phospholipase C (degrades cellular membranes);
4. Pyocyanin (generates reactive oxygen species).

Question 55

Mechanism of action and resistance of fosfomycin?

Answer 55

Fosfomycin blocks development of the NAM-NAG polymer backbone by inhibiting the enzyme MurA. Resistance stems primarily from efflux pumps or alterations to MurA that prevent fosfomycin binding.

Question 56

Mechanism of action of fidaxomicin?

Answer 56

Fidaxomicin is a macrolide antibiotic that inhibits the RNA polymerase sigma subunit, the component involved in binding promoter DNA so that transcription can be initiated.
Fidoxomicin is administered orally and has minimal systemic absorption, resulting in high concentrations in the colon and feces. It aslo has a narrow spectrum of activity, with a lesser effect on normal colonic flora than vancomycin.

Question 57

Treatment of Tinea pedis?

Answer 57

Variety of topical antifungals, including azoles (eg, miconazole, clotrimazole), allylamines (eg, terbinafine), and tolnaftate.

Question 58

Which antimicrobial medications associated with esophagitis?

Answer 58

Tetracyclines and clindamycin (due to their low pH and irritant effects on the distal eophagus).

Question 59

The human papillomavirus (HPV) has an affinity to infect which structures?

Answer 59

HPV infects basal epithelial cells thorugh small breaks in the skin or mucosal surfaces. It has a predilection for stratified squamous epithelium, which is found in the anal canal, vagina, and cervix. In the respiratory tract, the true vocal cords are the only area covered with stratified squamous epithelium (infants can acquire respiratory papillomatosis via passage through the birth canal of mothers infected with the virus).

Question 60

Functions of alkaline phosphatase and gamma-glutamyl transpeptidase?

Answer 60

1. Alkaline phosphatase is an enzyme that hydrolyzes and removes phosphate from other compounds (eg, proteins, enzymes).
   
   2. Gamma-glutamyl transpeptidase (GGT) helps convert glutathione to glutamate.

Question 61

Biopsy of anogenital warts will demonstrate …?

Answer 61

papillomatous epidermal hyperplasia with cytoplasmic vacuolization.